1. Field of the Invention
The present invention generally relates to tubular connectors. In particular, the present invention relates to an apparatus for connecting tubulars in such a way that the connection is prevented from becoming unmade in response to expansion of the tubulars in a wellbore. More particularly, the present invention provides a tubular connection using a thread insert to maintain sealing and mechanical integrity in a tubular connection during and after expansion.
2. Description of the Related Art
In order to access hydrocarbons in subsurface formations, it is typically necessary to drill a bore into the earth. The process of drilling a borehole and of subsequently completing the borehole in order to form a wellbore requires the use of various tubular strings. These tubulars are typically run downhole where the mechanical and seal integrity of the jointed connections are critically important in the original make-up of the tubulars, during expansion of the tubulars, and after expansion of the tubulars.
Typically threaded connections are used to connect multiple tubular members end-to-end. This is usually accomplished by providing tubulars that have a simple male to female threaded connection. The male end is generally referred to as a pin, and the female end as a box. The tubulars are connected, or xe2x80x9cmade-up,xe2x80x9d by transmitting torque against one of the tubulars while the other tubular is typically held stationary. Torque is transmitted in a single direction in accordance with the direction corresponding with connection make-up. Any torque applied to the joint in the make-up direction will have the effect of continuing to tighten the threaded joint.
When running tubulars there is sometimes a requirement to run jointed tubulars that will later be expanded by various types of expansion mechanisms. In some instances, tubulars are expanded by the use of a cone-shaped mandrel. In this manner, the tubular is expanded by forcibly moving the cone through the expandable tubular, deforming the steel beyond its elastic limit while keeping the stresses below the ultimate yield. Alternatively, another recent method of expanding tubulars relies on rotary expander tools that have been developed to operate in response to hydraulic forces. The rotary expander tool typically includes radially expandable members that are urged outwardly, through fluid pressure, from a body of the expander tool and into contact with a tubular therearound. As sufficient pressure is generated on a piston surface behind these expansion members, the tubular being acted upon by the expander tool is expanded past its point of elastic deformation. In this manner, the inner and outer diameter of the tubular is increased in the wellbore. By rotating the expander tool and by moving the expander tool axially in the wellbore with the expansion members actuated, a tubular can be expanded into plastic deformation along a predetermined length.
Tubulars to be later expanded are typically run downhole where the mechanical and seal integrity of the connections, or joint, are critically important both in the original and expanded state of the tubular. The current method of making-up expandable tubulars is by the design of modified threaded connections which can be applied and handled in the same way as conventional oil-field tubulars, i.e., stabbed into each other and screwed together by right hand or left hand rotation and finally torqued to establish the seal integrity. This method of connecting tubulars, though a reliable means of connecting non-expanding tubulars, is proving to be problematic when these tubulars are expanded. The reasons for this being mainly due to the changes in geometry of the connection during expansion due to the stresses applied at the threads, or joint area. For instance, conventional tubulars expanded at the joint may disengage allowing the lower tubing to fall into the wellbore.
It is well known and understood that during the expansion of solid wall tubulars, the material in the tubing wall is plastically deformed in more than just the circumferential sense. In order for a tubular to increase in diameter by plastic deformation, the material to make-up the additional circumferential section of wall in the larger diameter must come from the tubing wall itself either by reduction in wall thickness or by reduction in tubular length or a combination of both. In a plain wall section of the tubular this process will normally take place in a relatively controlled and uniform way. However, at the point of a threaded connection, or joint, the changes in wall section, which are required in order to form an expandable threaded connection, introduce very complex and non-uniform stresses during and after expansion. These during-expansion stresses significantly change the thread form and compromise the connection integrity both in terms of its mechanical strength as well as in terms of its sealing capability.
Additionally, the larger elastic deformation caused by the reduced sections of the tubing wall at the roots of the thread will introduce much higher stresses than in other areas of the expanded tubular. This in turn may lead to joint failure due to these stresses approaching or exceeding the ultimate strength of the tubing material or by introduction of short cycle fatigue caused by the cyclic nature of some expansion processes being applied at these high stress levels.
In non-petroleum applications, thread inserts, in particular helical thread inserts, are employed as a means for repairing stripped, worn, or damaged threads. Briefly, where the threads in a bore are stripped or worn, repair is effected by drilling out the bore to remove remnants of the damaged threads, thereafter tapping the drilled out bore and then inserting in the tapped bore an insert, the outer diameter of which is intimately engaged in the threads of the re-tapped bore, the inner diameter of the insert providing a threaded pin receiver portion of the same size and pitch as that presented by the original threading of the bore. In addition to this method, wherein an insert is seated into the recesses of a box thread, the present invention envisions threading the pin threads of a tubular with an insert prior to make-up with a second tubular.
The objective of the present invention is to resolve many of the problematic areas associated with the expansion of threaded connections in wellbore tubulars. Preferably, the present invention consists of placing a helical or spiral thread insert in engagement with the threads of a first tubular before make-up with a second tubular. The insert bridges any gaps that naturally exist between the threads of a pin and the mating threads of a box. During expansion of the tubular joint, the insert is plastically deformed along with the threads such that a constant wall thickness is maintained. This innovative concept of using plastic deformation of an insert between the mating threads of a jointed system being described herein provides the essential step to making this invention a practical and novel solution to expandable wellbore tubular connections.
The present invention may be summarized as an improvement on expandable wellbore tubular connectors. In accordance with the invention, a metal insert, preferably helical or spiral in nature, is placed in engagement with the threads of a first tubular before make-up with the threads of a second tubular.
In operation, an insert is engaged between the threads of the tubulars prior to make-up of the tubulars. This may be accomplished by engaging the metal insert around the external threads, commonly referred to as a pin, of a first tubular before make-up with the internal threads, commonly referred to as a box, of a second tubular. As such, it may be desirable to have deeper recesses or grooves, and/or shallower teeth, within the thread profile of one or both of the tubulars.
In the preferred embodiment, a helically wound wire insert is formed around a tapered lead, however a straight lead is an alternative. In the tapered lead embodiment, the insert itself may be formed to carry any number of similar, or dissimilar internal or external thread profiles. As expansion of the threaded joint occurs, the wire insert will plastically deform within the area between the pin-threads and the box-threads to seal any gaps occurring in the thread profile due to internal expansion. The expansion of the insert may occur due to stretching, or drawing of its circumferential length as the thread connection is expanded, or through slippage, relative to the encapsulating box and pin profiles.
In another embodiment, the insert would be made from a work-hardenable corrosive resistance alloy. Additionally, the wire insert could be coated with Teflon, or some other inert sealing medium known to those in the arts. Such a coating would provide increased sealing benefits.